The present invention relates to a motor with permanent magnets; and the motor can realize an operation of high efficiency and low noise. More particularly, a uniquely shaped air gap between teeth of a stator and a rotor of the motor lies at the heart of the present invention.
FIG. 8 is a cross section of a conventional motor with permanent magnets.
In FIG. 8, stator 580 comprises the following elements:
six units including six teeth 581 with windings (not shown) and six yokes 582 connected to respective teeth 581; and these six units being coupled with each other at junctions 583 thereby forming a ring-shaped stator.
Wires can be wound on respective teeth 581 independently, so that the normal windings on teeth are achievable. As a result, a winding in a high density with less wire-wound-resistance is obtainable.
Rotor 590 comprises the following elements:
rotor core 591 shaped in an approx. cylinder;
shaft 594 disposed at the center of rotor core 591;
four pieces of permanent magnets 592 mounted on an outer wall of rotor core 591; and
tube 593 made of e.g. thin stainless steel and covering permanent magnets 592 for preventing the magnets from scattering.
In such a conventional motor as discussed above, the air gaps between respective teeth 581 and rotor 590 have approx. the same width as shown in FIG. 8.
Rotating magnetic field, produced by electric current travelling through the windings of stator 580, rotates rotor 590 on shaft 594. The rotating magnetic field is produced in this way: Adjacent teeth have different phases with each other and the stator""s windings are thus three-phase-windings. In a regular operation, a driving circuit detects a position of the rotor and switches phases to be powered sequentially. In this case, each phase is powered every 120 electric angle, and yet one phase always remains non-powered, so that the rotating magnetic field is produced.
In the conventional motor with permanent magnets discussed above, opposite magnetic fields are produced between the adjacent teeth of active two pairs of windings out of six teeth. Therefore, magnetic flux at slot-open-section 584 is vulnerable to short-circuit. The slot-open-section is a clearance between the tips of the adjacent two teeth, and an opening section of a slot formed by the adjacent two teeth.
FIG. 9 shows Maxwell""s stress distribution produced at the air gap in the motor discussed above. The arrow marks show. the stress distribution. The arrow marks indicate the distribution of forces which the magnetic flux at the air gap supplies to the stator. When permanent magnet 5929 is polarized S pole, electric current travels through the windings so that tooth 5811 is magnetized S pole and tooth 5812 is magnetized N pole. Then rotor 590 rotates in rotating direction R as shown in FIG. 8. In this case, strong attraction is produced between magnet 5929 (S) and tooth 5812 (N), thus stress 587 produced therebetween becomes greater discontinuously.
FIG. 10 shows the way the magnetic-flux-density at the air gap around a slot-open in the motor changes with respect to a rotational angle of the rotor. In other words, FIG. 10 illustrates the time-based change of the magnetic-flux-density at the air gap around slot-open 584 in FIG. 8. In FIG. 10, the X-axis represents a rotational angle (mechanical angle) of rotor 590 in rotating direction R. When the magnetic-flux-density takes positive values, the rotor is polarized N pole, and when the density takes negative values, the rotor is S pole. The magnetic-flux-density seems to vary smooth; however, it rises sharply in 20-degree range on rotational angle 45-degree. This sharp change corresponds to stress 587 shown in FIG. 9, where stress 587 becomes large discontinuously, and this causes vibrations and noises in the motor operation.
The present invention addresses the problems discussed above, and aims to provide a motor having air-gap, which is situated between the teeth of the stator and the rotor, so that noise lowers and withstanding force against demagnetization grows.
The motor of the present invention comprises the following elements:
(a) a stator including a plurality of teeth on which wires are wound and yokes coupling the teeth; and
(b) a rotor having permanent magnets and facing to the stator via an air-gap.
A part of the air-gap at rotating tail of the teeth facing to the rotor is formed to be larger than other parts of the air-gap. This structure prevents the magnetic-flux-density at the air-gap from sharply rising, thereby lowering vibrations and noises.